The ryanodine (RY) receptors in beta-cells amplify signals by Ca2+-induced Ca2+ release (CICR). The role of CICR in insulin secretion remains unclear in spite of the fact that caffeine is known to stimulate secretion. This effect of caffeine is attributed solely to the inhibition of cAMP-phosphodiesterases (cAMP-PDEs). We demonstrate that stimulation of insulin secretion by caffeine is due to a sensitization of the RY receptors. The dose-response relationship of caffeine-induced inhibition of cAMP-PDEs was not correlated with the stimulation of insulin secretion. Sensitization of the RY receptors stimulated insulin secretion in a context-dependent manner, that is, only in the presence of a high concentration of glucose. This effect of caffeine depended on an increase in [Ca2+]i. Confocal images of beta-cells demonstrated an increase in [Ca2+]i induced by caffeine but not by forskolin. 9-Methyl-7-bromoeudistomin D (MBED), which sensitizes RY receptors, did not inhibit cAMP-PDEs, but it stimulated secretion in a glucose-dependent manner. The stimulation of secretion by caffeine and MBED involved both the first and the second phases of secretion. We conclude that the RY receptors of beta-cells mediate a distinct glucose-dependent signal for insulin secretion and may be a target for developing drugs that will stimulate insulin secretion only in a glucose-dependent manner.

No abstract

Insulin release is inhibited by adrenergic alpha-2 agonism in normal beta-cells. To test whether the inhibitory response to noradrenaline is modified by transplantation, we studied insulin release from freshly isolated islets and from syngeneic islets transplanted under the kidney capsule of non-diabetic C57BL/6 mice. When perifused in vitro, fresh islets, as well as grafts harvested 1 or 3 weeks after transplantation, reacted to 2.5 mumol/l noradrenaline with a complete inhibition of insulin release induced by 16.7 mmol/l D-glucose. In contrast, islet grafts harvested after 6, 12, or 21 weeks exhibited a conspicuous insulin secretory response to 16.7 mmol/l glucose in the presence of 2.5 mumol/l noradrenaline. Also a concentration of 0.25 mumol/l, noradrenaline inhibited the glucose-induced insulin release from fresh islets but not from 6-week-old islet grafts. It is concluded that transplantation under the kidney capsule induces a decreased inhibitory responsiveness to noradrenaline in islet grafts.

To elucidate the adrenergic responsiveness of transplanted pancreatic islets, normal BALB/c mice received 150 syngeneic islets under the left kidney capsule. After 12-40 weeks, the grafts were removed and compared with untransplanted islets by an in vitro perifusion technique. Noradrenaline (NA), 3 mumol/l, completely inhibited glucose-stimulated insulin release from untransplanted islets but not from grafts, whether or not the beta adrenergic blocker, L-propranolol, was present. UK-14,304, an alpha 2-specific adrenergic agonist, inhibited glucose-induced insulin secretion from untransplanted islets by 80-92% at 0.1 or 1 mumol/l, and by 35-56% at 5-10 nmol/l. Insulin secretion from islet grafts was also markedly inhibited by 0.1 or 1 mumol/l, but not by 5 or 10 nmol/l, UK-14,304. It is suggested that the diminished adrenergic inhibition of insulin release from islet grafts reflects an altered function of the alpha 2 adrenoceptors on the beta-cells.

To test whether the transplantation of pancreatic islets affects their basic functions, collagenase-isolated mouse islets were inserted under the left renal capsule of recipient animals. After various periods of time, grafts were removed from the kidney and examined for insulin content and secretory dynamics in a perifusion system. During syngeneic (C57BL/6, BALB/c) or subsyngeneic (NMRI) intrastrain transplantation, the graft insulin content fell drastically during the first week and stayed low for at least 6 weeks; first-phase secretion in general appeared suppressed. Immunosuppression by cyclosporin A had little effect on (sub)syngeneic grafts but markedly improved the performance of allotransplants. Daily injections of the calcium antagonist, verapamil, enhanced the insulin secretory responses of isolated grafts, whether (sub)syngeneic or allogeneic. In syngeneic and subsyngeneic grafts, the potentiating effect of acetylcholine on glucose-induced insulin release was markedly diminished, whereas that of caffeine was not. Transplanted islets also exhibited a subnormal responsiveness to the inhibiting action of noradrenaline. It is concluded that chronic denervation and transplantation of pancreatic islets may cause fundamental changes in the beta-cell responses to physiological regulators of insulin release.